Provision of IMS services via circuit-switched access

ABSTRACT

The present invention proposes a solution for providing IMS services to users having circuit-switched controlled terminal being not adapted to provide IMS services to the users. In particular, it is proposed, in order to allow IMS to take the full call and service control, to place a user agent being responsible for the user ported to the IMS in a new node type called Mobile Access Gateway Control Function (MAGCF). This new node combines the logical functionality of a cellular switching center and the logical functionality of IMS. Further it is proposed to enhance a user&#39;s register, like HLR, to provide also information about the availability of a MAGCF node and about whether a particular user is enabled to use the MAGCF functionality.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to providing IMS service to a user havingcircuited-switched controlled user's equipment.

BACKGROUND

Third Generation (3G) Networks such as UMTS (Universal TelecommunicationNetwork) and CDMA 2000 provide high-speed wireless Internet access tomobile users over a wide coverage area. For the 3G networks the IPMultimedia Subsystem IMS has been defined to provide cellular access tothe services of the Internet in order to support telephony andmultimedia services. The IMS uses packet-switched technology, inparticular IP-network and other IETF protocols for provision ofservices. The 2nd Generation networks, like GSM, provide voice based ona circuit-switched technology. The strength of IMS is the provision ofenhanced Services, for example multimedia services combining voice anddata. Further, the usage of IP-network as a single underlying standardallows an easy and fast service deployment.

A Session Initiation Protocol SIP has been chosen in IMS for signallingbetween the user's equipment UE and the IMS as well as between thecomponents within the IMS. The IMS uses SIP also to complete voice andmultimedia calls in the Internet. In order to be able to use the IMSservice, the communicating user's equipment has to support IMS, whichmeans SIP has to be implemented in the user's equipment.

In the following simplified network architectures of IMS is described.In particular the nodes being involved in provision of service in IMSarchitecture are mentioned.

The components of the IMS system are the Call Session Control Function(CSCF), the Media Gateway (MGW)/Media Gateway Control Function (MGCF),the Home Subscriber Register (HSR), the Application Server (AS), and theTransport Signalling Gateway (T-SGW).

The CSCF acts as a call server and handles call signalling, it supportsand controls the multimedia sessions and performs address translationfunctions. The CSCF can be functionally decomposed to S-CSCF, I-CSCF andP-CSCF. The Proxy-CSCF (P-CSCF) is the first contact point in a visitedIMS network and it provides authorization of bearer resources, furtherit forwards a SIP register request received from the User Equipment UEto an I-CSCF determined using the home domain name, as provided by theUE. In the opposite direction it forwards the SIP request or response tothe UE. Moreover the CSCF forwards SIP messages received from the UE toa SIP server (S-CSCF) whose name the P-CSCF has received as a result ofthe registration procedure.

The Interrogating-CSCF (I-CSCF) is the contact point within anoperator's network for all connections destined to a subscriber of thatnetwork operator, or a roaming user currently located within thatnetwork operator's service area. There may be multiple I-CSCFs within anoperator's network. The main function performed by the I-CSCF isassigning a S-CSCF to a user performing SIP registration. The ServingCall Session Control Function (S-CSCF) is the node that performs thesession management for the IMS network. There can be several S-CSCFs inthe network. The main functions of S-CSCF include: acceptation ofregistration requests from UE, interacting with services platforms forthe support of services. Further it provides endpoints with serviceevent related information (like for example notification oftones/announcement together with location of additional media resources,billing notification).

The Home Subscriber Register HSR is the centralized subscriber database.The HSR interfaces with the I-CSCF and the S-CSCF to provide informationabout the location of the subscriber and the subscriber's subscriptioninformation. The HSR is responsible for holding the following userrelated information: user identification, numbering and addressinginformation, user security information for authentication andauthorization. The HSR supports the user registration, and storesinter-system location information.

The IMS supports several nodes for inter-working with legacy networks.These are the Media Gateway Control Function (MGCF) and the TransportSignalling Gateway (TSGW).

The MGCF performs protocol conversion between cellular call controlprotocols and IMS protocols. For example, the MGCF receives a SIPmessage from the CSCF and converts it into appropriate ISUP messages andsends it to the Transport Signaling Gateway. Thus, the primary functionof MGCF is to convert signalling information from one format to anotherin uplink and downlink direction. In UMTS this will predominantly bebetween Pulse Code Modulation (PCM) in the PSTN and an IP based format.

The T-SGW is the signaling end point in the case of interworking withPSTN/legacy networks. It maps call-related signaling protocols from/toPSTN on an IP bearer and sends it to/from the MGCF. The T-SGW convertsthe lower layers of SS7 into IP.

As already mentioned the UMTS system allows mobiles operating in packetmode to establish voice calls using SIP as the signalling protocol. TheSIP messages are sent to communicate the request to the Call SessionControl Function (CSCF) in the IMS. In this case, the data istransmitted as packets throughout the UMTS network. However in order toaccess any service in IMS the user has to perform a registrationprocedure in the IMS system. Said registration procedure is performed bymeans of an user agent being implemented in the user's equipment.

Thus, the IMS has been deployed for the 3G networks for provision ofservices using packet-switched technology with SIP as applied signallingprotocol. However, currently the major numbers of user's equipment donot support IMS technology with SIP as signalling protocol for voiceservice, since said user's equipment is adapted for a circuit-switchedcontrolled domain. Thus, for the access to the IMS an adaptation of theuser's equipment is necessary.

SUMMARY AND DESCRIPTION OF THE INVENTION

Therefore, it is an object of the present invention to provide asolution for providing IMS services to user's equipment operating incircuit-switched controlled domain.

The invention is disclosed in the independent claims. Advantageousembodiments are described in the dependent claims being disclosed in thecorresponding parts of the description.

According to the present invention it is proposed to provide an AccessGateway Node comprising a circuit-switched function, a user agent and anexchange interface. The circuit-switched function has all the functionsbeing required to exchange signalling with the cellular-switchedcontrolled network, in which a user is located. Further thecircuit-switched function has the ability to exchange information with aregister being located in the circuit-switched controlled domain.Preferably said register is a Home Location Register HLR, which providesIP multimedia system parameter to the Access Gateway Node. Saidparameters are used according to the present invention for registrationthe user in the IMS system. In order to interact with the IMS thereceived data are provided over the exchange interface to the useragent.

In one embodiment it is proposed that the IP multimedia system parameteris an indication that a subscription of the circuit-switched controlleduser's terminal has been ported to the IP multimedia system. In thiscase the Access Gateway Node is responsible to provide the IP multimediasystem parameters which are required to register the circuit-switchedcontrolled user's terminal in the IP multimedia system.

In a further embodiment it is proposed that the received IP multimediasystem parameter, received from the register includes already a list ofparameters required to register the circuit-switched controlled user'sterminal in the IP multimedia system.

According to one of the embodiments of the present invention, theexchange interface is adapted to deliver the IP multimedia systemparameter provided by the circuit-switched function to the user agentbeing responsible for the registration of the user in the IP multimediasystem or for updating a current registration. However in order toexchange this kind of message between two different systems it isproposed to provide a translation mechanism between circuit-switchedcontrolled signalling and IMS controlled signalling, in particular inview of the different signalling protocols and their requiredparameters.

Further it is proposed that the circuit-switched function is adapted toreceive mobility related notification from a mobile switching node towhich the circuit-switched controlled user's terminal is attached forhandling the provision of an access to the IP multimedia system. Thusfor example in a GSM network a signalling mechanism is available toinform that the user is still attached to the network, although the userdoes not perform any calls. According to the present invention it isproposed to provide this kind of information to the IP multimedia systemin order to keep the user's registration alive.

Further it is proposed to provide a register located in acircuit-switched controlled domain and communicating with thecircuited-switched function, which is located in the Access GatewayNode. Preferably said register is a Home Location Register as alreadyknown in GSM providing data of all users having subscription in anetwork, for which the HLR is responsible. Said register comprises areceiver entity adapted to receive a request message regarding handlinga circuit-switched controlled user's terminal. The message might be forexample an update message when a user changes the serving areas. In thiscase a corresponding update of user's data is to be performed in theregister in order to be able to locate the user.

Further the register comprises a provision module adapted to provideinformation about the functionality of the circuit-switched function.According to the present invention the circuit-switched function eitheris enhanced with the user agent functionality, which means that thecircuit-switched function is part of the Access Gateway Node or thiskind of functionality is not supported, which means in case of GSM thecircuit-switched function is for example the well known MSC. It isproposed to store this information in a place being accessible by theregister. Another embodiment might be that an indication about supportedfunctionality of the circuit switched function is included in a messagesent from said circuit-switched function to the register. In this casethe task of the provision module is to extract the information from thereceived message. Said message might be either a separate message or therequired information might be included in any existing and suitablemessage, like for example in an update message.

Moreover it is proposed that the register comprises a user's parametermodule adapted to provide user's IP multimedia system parameterapplicable in the IP multimedia system in case the circuit-switchedfunction is part of the Access Gateway Node.

Further it is proposed that the register comprises a sender entity forsending the user's IP multimedia system parameter to the wirelessswitching function.

Further it is proposed that the register further comprises a trackingmodule adapted to keep track on a serving Access Gateway Node and on amobile switching node to which the circuit-switched controlled user'sterminal is attached. This might be for example the case when a user isin a visited network, which does not have the Access Gateway Nodes. Inthis case the user is attached to a usual circuit-switched function,like for example MSC. However the IMS provision is performed by means ofthe Access Gateway Node located in the home network. Thus, it isproposed that the register keeps track on both nodes.

Further it is proposed that the register node further comprises aselection unit adapted to select a new serving Access Gateway Node incase the circuit-switched controlled user's terminal leaves a servingarea of a present Access Gateway Node. The register might have forexample a list with the available Access Gateway Nodes and according tothe location of the user one is selected.

Further it is an advantage of the present invention that the registernode comprises also a modification module adapted to modify the user'sIP multimedia system parameter before sending it in case thecircuit-switched controlled user's terminal is served by a mobileswitching node. As above already mentioned when a user is to be handledby a well known MSC than it is proposed to send modified user's IPmultimedia system parameter, which means that although the user isported to IP multimedia system a usual circuit-switched service is to beprovided to the user.

Thus, according to the present invention the complete IP multimediaparameter is sent only to an identified Access Gateway Node. Otherwisesaid parameters are to be modified since not all of the parameters areapplicable for the circuit-switched function. For example it is proposedto remove services from the user's IP multimedia system parameter whichare only handled in the IP multimedia system.

Further it is proposed to have a method which is to be carried out inthe Access Gateway Node. Thus, it is proposed that a circuit-switchedfunction receives from a circuit-switched controlled user's terminal aregistration message, and upon receipt of this message an interactionwith a register node for providing user's IP multimedia system parameteris performed. When a message including the user's IP multimedia systemparameter from a server is received, said circuit-switched functionprovides the received user's IP multimedia system parameter to an useragent being also a part of the Access Gateway Node. The user agent usesthe user's IP multimedia system parameter to register thecircuit-switched controlled user's terminal in the IP multimedia system(IMS).

Further it is also proposed to have a method which is to be carried outin the register. In a first step the register receives a request messageregarding a circuit-switched controlled user's terminal from thecircuit-switched function for handling the circuit-switched controlleduser's terminal. Further information is provided about a functionalityof the circuit-switched function sending the request message. Thefunctionality indicates whether the circuit-switched function is part ofan Access Gateway. In such a case user's IP multimedia system parameterapplicable in the IP multimedia system are provided to thecircuit-switched function. Finally a message is sent to thecircuit-switched function with the user's IP multimedia systemparameter.

The advantage of the present invention is that it provides a smoothmigration from a circuit-switched domain to a packet-switched domain. Inparticular for a user with a circuit-switched adapted equipment asolution is given to use IMS service.

Further advantageous embodiments are described in the dependent claims.

In the following preferred examples of the present invention shall bedescribed in detail, in order to provide the skilled person withthorough and complete understanding of the invention, but these detailedembodiments only serve as examples of the invention and are not intendedto be limiting. The following description shall make reference to theenclosed drawings, in which

FIG. 1 shows a schematic representation of an architecture of AccessGateway Node according to the present invention, and

FIG. 2 shows a schematic representation of an architecture of the AccessGateway Node according to the present invention,

FIG. 3 shows a flowchart of an embodiment of the present invention for amethod being to be performed on the Access Gateway Node,

FIG. 4 shows a schematic embodiment for IMSI Attach in MAGCF and IMSregistration,

FIG. 5 shows an embodiment of a signalling sequence for performing IMSIAttach in MAGCF and IMS registration,

FIG. 6 shows a schematic embodiment for handling a user roaming betweentwo MAGCFs,

FIG. 7 shows an embodiment of a signalling sequence for performing ahandling of a user roaming between two MAGCFs,

FIG. 8 shows a schematic embodiment for handling a user roaming betweena MAGCF and a MSC,

FIG. 9 shows an embodiment of a signalling sequence for performing ahandling of a user roaming between a MAGCF and a MSC,

FIG. 10 shows a schematic embodiment for handling a user roaming betweentwo MSCs, wherein the user is already registered in a IMS system,

FIG. 11 shows a schematic embodiment for handling a user roaming betweena MSC and a MAGCF,

FIG. 12 shows an embodiment of a signalling sequence for performing ahandling of a user roaming between a MSC and MAGCF.

It should be noted that the term “entity”, “node”, “module”, in thecontext of the present invention refers to any suitable combination ofhardware and software for providing a predetermined functionality in thecommunication network. In this way, said terms generally refers to alogical entity that can be spread out over several physical entities,but can also refer to a physical entity located in one physicallocation, if no explicit definition is given.

It should be noted that the term “user” in the context of the presentinvention refers to circuit-switched controlled user equipment, whereinsaid user equipment is a combination of hardware and software. Howeverin the following description the terms “user” and “user equipment”should be seen as having the same meaning, if it is not stateddifferently.

Preferably, the communication network is a mobile communication network,e.g. is a wireless communication network operating according to GSM,GPRS (General Packet Switched Radio) or UMTS (Universal Mobile TelephoneSystem) or any 3G system like for example EDGE, CDMA2000.

In the following a preferably embodiment according to FIG. 1 ispresented. FIG. 1 shows a schematic representation of an architecture ofan Access Gateway Node, 10 according to the present invention. Inparticular in FIG. 1 the Access Gateway Node, 10 is depicted whichcommunicates on one side with the IP multimedia system, which will becalled in the following IMS, 11 and on the other hand with thecellular-switched controlled network, 12, like for example GSM. In theforegoing description a node called MAGCF Mobile Access Gateway ControlFunction represents an embodiment of the Access Gateway Node, 10. TheAccess Gateway Node comprises a circuit-switched function 101, a useragent 102 and an exchange interface 103.

It is to be noted that although the Access Gateway Mode, 10 is depictedin FIG. 1 between the network 11 and the network 12, this locationshould not be seen as any restriction for the present invention. TheAccess Gateway Node is to be located in any suitable place. According toone of the embodiments it is proposed to place said node in thecircuit-switched domain, 12.

The circuit-switched function 101 is preferably a function fulfillingthe functionality of MSC. Thus, it has all the functions being requiredto exchange signalling with the cellular-switched controlled network, 12in which a user is located, wherein the communication with the user isperformed over a radio interface, not depicted in FIG. 1. Saidconnection might be performed by means by any suitably signallingprotocol, in case of GSM this might be the well-known Mobile ApplicationProtocol MAP or ISUP or BICC. Thus, the task of the circuit-switchedfunction 101 is to provide exchange signalling for a circuit-switchedcontrolled user's terminal located in the circuit-switched controlleddomain CS, as it is performed currently in GSM by means of MSC with allits functions. Further the circuit-switched function 101 has the abilityto exchange information with a register 121 being located in thecircuit-switched controlled domain, interface 13. Preferably saidregister is a Home Location Register HLR, which provides IP multimediasystem parameter to the Access Gateway Node. Said parameters are usedaccording to the present invention for registration the user in the IMSsystem. In order to interact with the IMS the received data are providedover the exchange interface 103 to the user agent 102. Preferably, a SIPagent might perform the task of the user agent 102. This includes forexample the registration of the user or updating of the user's data inthe IMS. The communication for this purpose is performed by means of thelink 14 according to FIG. 1

FIG. 2 represents an embodiment for the realisation of a register 121located in a circuit-switched controlled domain, CS, 12 andcommunicating with the Access Gateway Node 10. Preferably said registeris a Home Location Register as already known in GSM providing data ofall users having subscription in a network, for which the HLR isresponsible. According to the present invention it is proposed to extendthe register in a way to be able to provide IP multimedia systemparameter to the Access Gateway Node, besides its ability to communicatewith a circuit-switched function 101, like for example in case of GSMwith the MSC. According to FIG. 2 said register 121 comprises a receiverentity 21 adapted to receive a request message regarding handling acircuit-switched controlled user's terminal, 201. The messages might befor example an update message when a user changes the serving areas. Inthis case a corresponding update of user's data is to be performed inthe register in order to be able to locate the user. In case of GSM aMAP:Location update message is sent from the MSC to the HLR. However,this is only one preferably embodiment for the implementation of themessages.

Further the register 121 comprises a provision module 22 adapted toprovide information about the functionality of the circuit-switchedfunction 101. According to the present invention the circuit-switchedfunction either is enhanced with the user agent functionality, whichmeans that the circuit-switched function is part of the Access GatewayNode or this kind of functionality is not supported, which means in caseof GSM the circuit-switched function is the well known MSC. Thus, it isthe task of the provision module to provide this kind of information.There might be different ways of suitable and preferable realisations.For example this kind of information might be stored in a place beingaccessible by the register and not in the register itself. This might befor example a list with all the circuit-switched functions and anindication whether said circuit-switched function is part of the AccessGateway Node. The task of the provision module is in this case todeliver the information from the list. Further embodiment might be thatan indication about supported functionality of the circuit switchedfunction is included in a message sent from said circuit-switchedfunction to the register. In this case the task of the provision moduleis to extract the information from the received message. Said messagemight be either a separate message or the required information might beincluded in any existing and suitable message, like for example in anupdate message.

Moreover it is proposed that the register 121 comprises a user'sparameter module 23 adapted to provide user's IP multimedia systemparameter applicable in the IP multimedia system in case thecircuit-switched function is part of the Access Gateway Node. Preferablythe IP multimedia system parameters are to be stored together with otheruser's related data. However it might be also suitable to store onlyindication that the user is ported to the IP multimedia system. The useris ported to IP multimedia system, if the user has made an activedecision to end a current circuit-switched contract with thecorresponding service provider and has signed a new contract for usingIP multimedia Services. Another option might be that the operatordecides to port all of the subscribers to the IP multimedia Servicesproviding still some subset of the old circuit-switched services.

Further parameters which might be required during different proceduresmight be delivered from the appropriate places, like for example anadditional data base might be provided which is to be accessible by theuser agent 103.

Further it is proposed that the register comprises a sender entity 24for sending the user's IF multimedia system parameter to the wirelessswitching function over the interface 202.

In respect to FIG. 3 an embodiment of the present invention for a methodto be performed on the Access Gateway Node is presented. In step 31 acircuit-switched function located in the Access Gateway Node receivesfrom a circuit-switched controlled user's terminal a registrationmessage. That implies that the user has a circuit-switched terminalwithout support for the IMS services. According to the well-knownprocedure the wireless switching function being, according to thepresent invention, part of the Access Gateway Node interacts with aregister node for providing user's IP multimedia system parameter, 32.In the subsequent step, 33 the circuit-switched function receives therequested IP multimedia system parameters and provides them to a useragent located in the Access Gateway Node, 34. Upon receipt of saidparameter the user agent initiates a procedure for registration the userin the IP multimedia system (IMS), 35.

In the following a basic concept of the present invention is described.According to the present invention it is proposed, in order to allow IMSto take the full call and service control, to place a user agent beingresponsible for the user in a new node type called MAGCF being anembodiment of the above mentioned Access Gateway Node. This new nodecombines therefore the logical functionality of a cellular switchingcenter and the logical functionality of IMS. In particular it isproposed that the MAGCF comprises well-known functionality like forexample a MSC-S being serving MSC for a roaming user, a GMSC-S forterminating calls to a roaming user in a network without MAGCFfunctionality, a MGCF performing protocol conversion between cellularcall control protocols and IMS protocols, a P-CSCF forwarding SIPmessages from the user to IMS and from IMS to the user, a User Agentfulfilling the IMS functionality and handling on behalf of the user.

The basic concept is that the MAGCF hosts the user agent of thesubscriber and registers the subscriber having a cellular controlledaccess in IMS. Therefore, the MAGCF is the roaming anchor point for thecellular access towards IMS. In the following different embodiments forthe realization of the anchor point are described. For example anembodiment is given describing changing of a service MSC-S while roamingin a home network. In this case the MAGCF roaming anchor point is movedto the new MSC-S. Further, when roaming into a visited network withoutsupport for MAGCF, the last responsible MAGCF is kept as anchor point.In all embodiments it is to be ensured that all originating andterminating calls of the subscriber are routed via the responsibleMAGCF.

In general it might be said that the MAGCF interworks thecircuit-switched protocols towards IMS protocols and vice versa.

In the following an embodiment of the present invention is described inrespect to FIG. 4, presenting the user's registration procedure,including an attach procedure.

FIG. 4 depicts a cellular capable user equipment UE communicating withthe MAGCF over a UTRAN/GERAN access network. The MAGCF node according tothe present invention has functionality to communicate with thecellular-switched controlled system and with the IMS system. The IMSsystem comprises I-CSCF, S-CSCF and HSR nodes. According to the presentinvention said MAGCF node has the MSC functionality, like MSC-S and theIMS functionality in form of user agent UA. The MAGCF has therefore thecapability to communicate with the HLR and with the IMS. Further thereis also the IMS Access Server IMS AS for provision of IMS servicescommunicating with HSR and S-CSCF. The sequence of the messages exchangeis depicted by means of lines provided with arrows. The dotted lines areprovided to show the signalling connections between the nodes.

In the following traffic case is described, when a user switches on thecircuit-switched controlled user equipment UE in a home network. In thefirst step, 401 UE sends an IMSI attach message to the MAGCF in order tobecome reachable via the circuit-switched controlled access. Since thisis a new subscriber being not registered in the MAGCF, the responsibleHLR is contacted to inform about the location update, 402. Preferablythe HLR has information that the received information is from a MSCbeing enhanced with the MAGCF functionality. In this case of a staticlist being administrated by the HLR, no changes on the interface betweenMSC and HLR has to be implemented, that means that a standard protocolfor communication, like for example the MAP protocol might be used.However optionally in step 402, the MAGCF can indicate to the HLR thatit has the MAGCF functionality, to differentiate itself from a MSCsupporting only cellular-switched functionality. However, this wouldimply changes to the standard MAP protocol. On the basis of the providedinformation, either by checking the indication in the MAP UpdateLocation message, or by checking a list with all MAGCF nodes, the HLRchecks whether the sending node has MAGCF functionality. In thefollowing step, the HLR checks the requesting subscriber. In order toprovide the IMS functionality to the users, said users has to be portedto the IMS system. In other words, the user has to either announceactively the change to the IMS system or the system might decide to portcellular users to the IMS system. A corresponding notification about theuser is to be stated in the HLR. In case a user is not ported, it isproposed to apply the standard behaviour for a cellular user like it isalready known. In case the user is ported to the IMS, it is proposedthat the HLR inserts the IMS related subscriber data into the MAGCFrequired to register said user to the IMS system, step 403. Preferablythe HLR sends all the parameters, which are usually stored at the SIMcard in the user's equipment. The MAGCF receives the message 403,wherein it is to be pointed out that the communication with the HLR isperformed by the circuit-switched functionality of the MAGCF, inparticular the MSC is involved therein. Upon receipt of the message fromthe HLR, the MAGCF checks at first whether the user supports the IMS, incase of supporting the MSC-F contacts the user agent in order toregister and to subscribe the subscriber in the IMS system, steps 404and 405. Preferably the applied protocol for the registration and forthe subscription purpose is the SIP protocol, in this case also the useragent has SIP functionality implemented. During the IMS registration,the user agent acts on behalf of the subscriber. All steps necessary forregistration like for example, the authorization of the users, areperformed by the means of the IMS entities integrated in the MAGCF. Theresult of the registration is that the MAGCF stores the S-CSCF addressand following the IMS rules for registration, the S-CSCF stores theMAGCF address where the registered subscriber can be reached. Aftersuccessful registration, in step 405, the user agent in the MAGCFsubscribes in the S-CSCF to the registration status of the servedsubscriber. So if the registration status of the subscriber is changedin the S-CSCF, the MAGCF is to be informed.

As a consequence of the procedure according to FIG. 4, the subscriber isattached in the MAGCF from the cellular controlled side and registeredin the IMS. The same procedure is presented in more detail in thesignaling sequence in respect to FIG. 5. The same nodes as in FIG. 4 aredepicted.

Currently in order to access circuit-switched service a registrationprocedure of the user in the circuit-switched network is to beperformed. Said registration procedure might be performed duringlocation update procedure. Thus, in case a user changes the location a24.008 IMSI attach message is sent to the responsible mobile switchingcenter MSC, which derives the Home Location Register HLR address fromthe received IMSI. Thus, in the first step the UE sends the 24.008message Location Update to the MAGCF, indicating the location updatetype, namely IMSI attach, to initiate the registration procedure in thecellular system. By means of a MAP protocol being the applicationprotocol employed for the dialogues between the MSC and HLR, the HLR iscontacted for authentification of the user. For this purpose the MAGCFsends MAP: Fetch triplets to the HLR in order to receive user'sparameters required for performing authentication. In case the user isauthorised, the procedure for updating the user's location by sendingthe message MAP: Update Location (IMSI) is sent. Subsequently, the HLRchecks whether the sending node has MAGCF functionality. This can bechecked either by receiving an indication in the MAP Update Locationmessage, or by checking a list being accessible by the HLR where allMAGCF nodes are listed. Preferably this list is to be administered inthe HLR. In the following step, the HLR checks the requestingsubscriber, whether the subscriber is ported to IMS. If the user is notported, standard behaviour for location update of a cellular user isapplied by sending MAP: Insert Subscriber Data in order to insert theuser to the MSC. However, if the user is ported to IMS, the HLR insertsthe subscriber data into the MAGCF by sending MAP: Insert SubscriberData. Additionally in the message, IMS related data, like informationrequired for the registration in the IMS system might be sent. Thismight be for example IMPI (IP Multimedia Private Identity), IMPU IPMultimedia Public identity, Home Register, and/or Authentication key.The IMPI and the IMPU are the private and public identity of thesubscriber, used together with the Authentication Key to register in theHome Registrar of the subscriber. In other words the HLR sends all theparameters to the MGCF, which are stored on the SIM in user's terminal.Finally, the message MAP:Update Location Accept is sent from the HLR tothe MAGCF, finalizing the registration procedure in the cellularcontrolled network.

After the interaction the HLR is completed, the user agent located inthe MAGCF registers the subscriber into IMS, indicating the reachabilityof the user via cellular-switched controlled access. Thus, during theIMS registration, the user agent handles on behalf of the subscriber.

As already mentioned the IMS uses SIP protocol for performing any kindof signalling, including the user's registration in the IMS system. Auser equipment needs to perform IMS service registration before it canset up a session. Through a successful registration a suitable S-CSCF inthe home network is assigned to the user in order to obtain the IMSservice. According to the present invention the User Agent located inthe MAGCF handles on behalf of the user. At first user's identity in theinitial register request is sent to the I-CSCF, SIP:Register.

For the IMS the user agent located in the MAGCF is transparent, whichmeans that the IMS does not notice that a user agent acts on behalf ofthe user and which means again, that the location of the user agent inthe MAGCF does not have any impact on the registration or subscriptionprocedure.

As aforementioned the I-CSCF is the contact point within an operator'snetwork for all connections destined to a subscriber of that networkoperator, or a roaming user currently located within that networkoperator's service area. The main function performed by the I-CSCF isassigning a S-CSCF to a user performing SIP registration. The S-CSCF isthe node that performs the session management for the IMS network.

Returning to FIG. 5 in order to find out whether an S-CSCF is alreadyassigned to the user, the I-CSCF contacts the HSR, Auth.Req., being adata base administrating all IMS users. Subsequently the I-CSCF receivesan answer with an address of a S-CSCF for contacting. The address mightbe either an S-CSCF already allocated to the user or if it is the firstregistration, for example after the user switched on the user equipment,the HSR returns a set of S-CSCF capabilities for selecting an S-CSCF. Onthe base of the capabilities and other available information an I-CSCFchooses an appropriate S-CSCF for the particular user. Then, the I-CSCFcontinues with the process by proxying the SIP:Register message to thechosen S-CSCF. The S-CSCF upon receipt of the message, starts theauthentication of the user. For this purpose a dialogue with the HSR isperformed for downloading authentication data. The HSR sendsauthentication data to the S-CSCF regarding the requesting user, so thatthe S-CSCF can properly authenticate the user. In order to verify theauthentication performed internally, the S-SCCF sends a 401 Unauthorizedmessage being forwarded via I-CSCF to the SIP receiver, whereinaccording to the present invention the SIP receiver is the user agent inthe MGACF. Therefore for the IMS nodes, like I-CSCF, HSR, S-CSCF theuser's functionality being shifted to the user agent according to thepresent invention, is transparent and said nodes do not notice anychange in the handling of the registration.

When the user agent receives the SIP 401 Unauthorised response, itrealises that there is a challenge included to deliver authenticationparameter stored locally. Thus, upon receipt of this message the useragent derives the authentication data in an appropriate way bycalculating a response and the user agent sends the second SIP: Registermessage to the S-CSCF. The same operation as for the first registrationrequest is performed and it results that the S-CSCF performs a checkingof the response with the already for the user stored authentication andif the verification is successful, then the S-CSCF sends a ServerAss.Req to the HSR for the purpose of informing the HSR that the user isnow registered and in order to download the user profile. The S-CSCFreplies with 200OK response and the MAGCF stores the S-CSCF address.Finally, following the IMS rules for registration, the S-CSCF stores theMAGCF address where the now registered subscriber can be reached. Thestoring of the MAGCF address is performed with receiving SIP: Subscribe(change of MAGCF address), which is also confirmed with the message200OK. After finalising the registration procedure in the IMS, a 24:008Location Update Ack is sent to the user indicating successful performedregistration.

Additionally in this embodiment it is proposed that after successfulregistration, the user agent in the MAGCF subscribes in the S-CSCF tothe registration status of the served subscriber. Thus, if theregistration status of the subscriber changes in the S-CSCF, the MAGCFis to be informed thereabout. For example in case of roaming as it isdescribed in the following embodiment, the MAGCF is to be informed thata user has left the responsibility area of said MAGCF.

The interaction with the IMS AS regarding third party registration isdepicted in FIG. 5 for the sake of completeness without any impact onthe invention.

In the following an embodiment describing roaming procedure in a homenetwork is described in respect to FIG. 6.

The structure and nodes in FIG. 6 are similar to the structure accordingto FIG. 4. A home network is a network, with which a user hassubscription for provision of services. Usually in a network a number ofMSCs is provided, wherein a MSC is responsible for users being locatedin location areas being assigned to the MSC. Changing of the responsibleMSC due to user's movement implies initiation of the roaming procedureaiming to register the user in a new MSC and de-register from the oldMSC by performing all the necessary updates in respect therewith in thecorresponding nodes.

Thus, FIG. 6 depicts a home network with two MAGCFs nodes, MAGCF61 andMAGCF62 and with a user roaming from MAGCF61 to MAGCF62, in step 600.

In this case the new MAGCF, MAGCF62 takes over the handling of thesubscriber. However the change of MAGCF also affects the IMSregistration, which is described in the following in respect to FIG. 6.

In step 601, after entering the new location area, the terminal sends alocation update request to the new MAGCF, MAGCF62. When receiving thismessage, the MAGCF62 identifies the subscriber to be new in itsresponsibility. Subsequently the MAGCF62 informs the HLR about theavailability of the user by sending for example MAP:Update Location,step 602. Similar as before mentioned, either the MAGCF indicates in theMAP message that it supports the MAGCF functionality, or the HLRdetermines the supported functionality of the MSC by checking a list inthe HLR. Upon receiving the location update message, 602, the HLRinforms the old MAGCF, MAGCF61 that the subscriber has roamed into a newMAGCF area. The HLR sends preferably a MAP Cancel Location message, 603,to the old MAGCF, MAGCF61. The receipt of this message means for a MAGCFthat it does no longer server the subscriber. However, the old MAGCFcannot remove the subscriber record yet, as it is still responsible forthe IMS registration. In step 604 the HLR inserts the subscriber datainto the new MAGCF using a similar approach as it is described inrespect to FIG. 4. Also the registration, step 605 and subscription 606to the IMS system are performed in the similar way. That means, that thenew MAGCF registers the subscriber into IMS, which results in changingthe address of the serving MAGCF in the S-CSCF changes, 605. Aftersuccessful registration, the user agent in the new MAGCF subscribes inthe S-CSCF to the registration status of the served subscriber, 606, sothat if the registration status of the subscriber is changed in theS-CSCF, the MAGCF will be informed. As a consequence of this approach,in step 607, the old MAGCF, MAGCF61 is informed by the S-CSCF that a newMAGCF is registered in the IMS. Upon receipt of the MAP Cancel Location603 and the Notification of IMS subscription change message 607, theMAGCF61 removes the subscriber record. The result is that the cellularswitched subscriber is now registered in IMS via the new MAGCF.

Optionally it is proposed to avoid sending the Notification of IMSsubscription change message 607. The receipt of the message might beonly issued if a MAGCF subscribes for receiving these kind ofnotifications.

In the following an embodiment describing roaming procedure in a homenetwork is described in respect to FIG. 7 presenting an exchange ofsignalling messages.

The nodes are similar as it is depicted and described according to FIG.5. Additionally a new and an old MAGCF are depicted. All the messagesare the same as in FIG. 5. The difference is that the HLR sends a MAP:Cancel Location message to the old MAGCF after performed registrationprocedure of a user in the cellular system. The receipt of this messagecauses that the old MAGCF suspends the MSC-S functionality. Also theregistration procedure in the IMS is similar as in case of IMSregistration as described in respect to FIG. 5. The difference is thatthe S-CSCF notifies the old MAGCF that the MAGCF address has changed bymeans of the SIP: Notify message, which is confirmed with 200 OKmessage. Finally a SIP:Subscribe (exp=0) is sent which means that theold MAGCF unsubscribes itself from receiving any further notificationabout changes in user's registration in IMS. After sending this messagethe old MAGCF goes to idle status regarding the unregistered subscriber.

While roaming, the subscriber may leave the area of the home network andenter a visited network, in which a user does not have any subscriptionfor providing any services. Depending on the capabilities of the visitednetwork, it may also support the MAGCF functionality in its MSC-S. Inthis case roaming is handled in the same way as the above describedroaming in the home network. The mechanism is the same, regardlesswhether it is the home or the visited network.

However, it may happen that the visited network does not support MAGCFfunctionality. In this case, special care is needed in order to servethe subscriber, as the visited network cannot take care of the IMSregistration. According to the present invention it is proposed to keepthe previously used MAGCF as a roaming anchor point. In the followingthis kind of network is called remote visited network

In the following an embodiment of the present invention is described inrespect to FIG. 8 showing the case of roaming from MAGCF to a MSC. Thenodes are similar to the nodes and connections therebetween as depictedin FIG. 6. The difference is that the new location area, to which a usermoves into, is served by a MSC without the MAGCF functionality. Thus,when entering the new location area, 800, the user terminal sends alocation update request to the new MSC/VLR, 801. When receiving themessage, the MSC/VLR identifies the subscriber to be new in the servingarea of said node. In the following step, 802 the MSC/VLR informs theHLR about the new subscriber. Hereby, there are again two possibilitiesof informing HLR whether the MSC supports the MAGCF functionality,namely either by sending such kind of information included in a messageor by checking HLR's internal list of MAGCF nodes. In the presentedembodiment, the result of the checking procedure is that the contactingMSC does not have the MAGCF functionality.

Therefore in step 803, the HLR informs the MAGCF that the subscriber hasroamed into a new MSC/VLR area. Preferably, the HLR sends a MAP CancelLocation message to the MAGCF. The receipt of this message means for theMAGCF that it is not responsible for serving the user in the cellularswitched controlled network anymore. However, the MAGCF cannot removethe subscriber record yet, as it is still responsible for the IMSregistration. Therefore it is proposed that the MAGCF stays as a roaminganchor point as long as the subscriber is roaming in the remote visitednetwork. The HLR inserts the subscriber data into the MSC/VLR as it iswell-known, 804. These data do not contain any new data being IMSspecific enhancements as it is the case in the previously presentedembodiments.

Further it is proposed to provide a data filter in the HLR for filteringsubscriber data being sent to the MSC/VLR in order to prevent executionof IMS services in the MSC/VLR. This when sending the message 804, forexample in form of the MAP operation Insert Subscriber data, the HLRfilters out all supplementary service subscriptions before sending thedata to the MSC/VLR, like Barring of all outgoing international calls(BOIC), which is a supplementary service, which prevents to conductinternational calls, when this service is active.

In the following signalling exchange sequence is presented in respect toFIG. 9 depicting the embodiment of a user's roaming from an old MAGCF toa new MSC/VLR.

The structure of FIG. 9 is similar to the structure of theaforementioned embodiments describing signalling sequences. The LocationUpdate in the HLR via MSC/VLR is the same as previously described. Bychecking the capabilities of the MSC from which the MAP: Update Location(IMSI) is received, the HLR notices that the MSC does not support theMAGCF functionality and therefore the HLR filters subscriber data bysending “not subscribed” for the supplementary services. As aconsequence, MAP:Insert Subscriber Data message is sent to the MSC/VLRfor informing the MSC of non-availability of all circuit-switchedservices for the user.

It is to be noted that the IMS registration in the S-CSCF is notaffected by roaming into a remote visited network. Moreover the oldMAGCF after receiving the MAP: Cancel Location only suspends the MSCfunctionality for the user, since this functionality is to be performedby the new MSC/VLR. However, the old MAGCF is still the anchor point forthe circuit-switched access towards IMS and interworks circuit-switchedprotocols towards IMS protocols. The telephony service as such and thetelephony supplementary services are still provided by IMS andespecially all incoming and outgoing calls are routed via the old MAGCF.

In the following a further embodiment is presented. When roaming in theremote visited network, the subscriber may enter the service area ofanother MSC/VLR. The change from the old MSC/VLR towards the new MSC/VLRwhile roaming is performed in the following described way according toFIG. 10.

As already mentioned, when entering a new location area, 1000, theterminal sends a location update request to the new MSC/VLR. Whenreceiving the message, the MSC/VLR identifies the subscriber to be newin its serving area, 1001. In step 1002 the MSC/VLR informs the HLRabout the new subscriber.

The HLR either receives or delivers from the internal data that theMSC/VLR sending the message does not have MAGCF functionality. In thesubsequent step, the HLR informs the old MSC/VLR that the subscriber hasroamed into a new MSC/VLR area. The HLR sends the message 1003 to theold MSC/VLR. The receipt of this message means for the old MSC/VLR thatit does no longer server the subscriber. The HLR inserts the subscriberdata into the new MSC/VLR, 1004. These data do not contain any new, IMSspecific enhancements due to the fact that the MSC/VLR does not supportany MAGCF functionality. However, an indication is sent to cause theMSC/VLR to route all originating call to the MAGCF. Also as in theforegoing embodiment explained, the subscriber data sent to the MSC/VLRare filtered, in order to prevent execution of services in the MSC/VLR.Also as in the above described embodiment the IMS registration in theS-CSCF is not affected by this roaming into a remote visited network.The difference to the previous embodiment is that the changing of theMSC/VLR does not affect the MAGCF; the location update is performedtransparent without involving the MAGCF.

The signaling sequence according to this embodiment is similar to thesignaling sequence as described in respect to FIG. 9, with the onlydifference that no MAP:Cancel Location message is sent to the MAGCF butinstead of this, said message is sent to the old MSC/VLR. Therefore nofigure presenting the signaling sequence is introduced.

In the following embodiment, in respect to FIG. 11, the scenario ispresented, when roaming in the remote visited network, 1100, thesubscriber may reenter the home network or another visited network thatsupports MAGCF functionality. When entering the area of an MAGCF, thismay be the current roaming anchor, or any other MAGCF.

The first steps, 1101, 1102 remain the same as in the previousembodiments. Thus, when entering the new location area, the terminalsends a location update request to the new MAGCF, 1101, and the MAGCFinforms the HLR about its new subscriber, 1102. The HLR informs the oldMSC/VLR that the subscriber has roamed into a new MAGCF area, 1103.Preferably by sending a MAP Cancel Location message to the old MSC/VLR.Receipt of this message indicates to the MSC/VLR that it does no longerserver the subscriber. Since the HLR has the information that the newMSC support the MAGCF functionality, the HLR inserts data into theanswer required for the registration in the IMS system, 1104. The newMAGCF registers the subscriber into IMS by changing the address of theserving MAGCF in the S-CSCF changes, 1105. After successfulregistration, 1106, the user agent in the new MAGCF subscribes in theS-CSCF to the registration status of the served subscriber. So if theregistration status of the subscriber is changed in the S-CSCF, theMAGCF will be informed. Since the old MAGCF was also registered to benotified in case of changes in the IMS subscription, the old MAGCF istherefore to be informed by the S-CSCF that a new MAGCF is registered.The old MAGCF can be unsubscribed from these notifications, 1107. Afterreceiving the message 1107, notifying of IMS subscription change, theold MAGCF removes the subscriber record. As a result of this procedure,the subscriber is now registered in the cellular system and in IMS bymeans of the new MAGCF.

The signalling sequence describing this embodiment in respect to FIG. 12is similar to the sequence for example in FIG. 7, the difference is thatthere is a new, an old MAGCF and an old MSC/VLR. Thus before performingthe registration in IMS MAP:Cancel Location to the old MSC/VLR is sentto de-register the user from the access in cellular system via the oldMSC/VLR. Afterwards the registration procedure in IMS is performed, asdescribed for example in respect to FIG. 5. Finally the old MSGCF isinformed by means of SIP:Notify(change of MAGCF address) about changesin user's status in IMS. Said message is acknowledged with 200OK to theS-CSCF and the consequence is that the old MAGCF is not anymoreresponsible for the user in the IMS system.

In the above described embodiments the roaming has been discussed indetails. However, on top of the procedures done when the subscriberactually roams, some procedures are triggered also for stationarysubscribers. Those events are for example the Re-Registration,De-Registration, 24.008 Location Update, 24.008 Periodic Location Updateor 24.008 IMSI Detach procedures, which are in the following mentioned.

The MAGCF has to initiate a SIP re-registration procedure towards theS-CSCF at expiry of the registration timer of the user agent. Theregistration is similar to the registration as aforementioned with thedifference that the IMS sets the time intervals for a newre-registaration. In case the re-registration is not performed theuser's registration is deleted. Preferably the MAGCF should checkwhether the UE has performed successful periodic location updaterecently.

The MAGCF has to initiate a SIP de-registration procedure at receptionof a 24.008 IMSI Detach from the UE. The procedure is equal to theregistration, but expires value is set to 0.

When receiving a 24.008 Location Update from the user equipment with anew Location Area Identifier LAI, this has to be stored in the VLR forthe case of paging. This procedure is similar to the case of roamingwithin the service area of the MAGCF.

Finally it is to be mentioned that the MAGCF has to initiate a SIPde-registration when a 24.008 IMSI detach message is received informingthat the user's terminal is switched off. The procedure is equal to theregistration, but expires value is set to 0.

The above-described embodiments are based on integration ofcellular-switched controlled users equipment, like it is provided in GSMor GPRS, into IMS services developed in connection with UMTS. However,the present invention is not only restricted to these networks. Afurther example might be CDMA2000 providing corresponding nodes as thenodes existing in GPRS or UMTS.

The invention claimed is:
 1. An Access Gateway Node (MAGCF) forproviding access to an IP multimedia system for a circuit-switchedcontrolled user's terminal located in a circuit-switched controlleddomain, wherein said node comprises: a circuit-switched functionconfigured to provide exchange signaling for a circuit-switchedcontrolled user's terminal, wherein said circuit-switched function isconfigured to interact with a home register located in a homecircuit-switched controlled domain to obtain from the home register, auser's IP multimedia system parameter, the user's IP multimedia systemparameter being an indication that a subscription of thecircuit-switched controlled user's terminal has been ported to the IPmultimedia system; a user agent configured to utilize the user's IPmultimedia system parameter to register the circuit-switched controlleduser's terminal to the IP multimedia system on behalf of thecircuit-switched controlled user's terminal, wherein the user agentregisters the circuit-switched controlled user's terminal to the IPmultimedia system without further signaling with the circuit-switchedcontrolled user's terminal; and an exchange interface configured toexchange signaling between the user agent and the circuit-switchedfunction.
 2. The Access Gateway Node according to claim 1, wherein theuser's IP multimedia system parameter is a list of parameters requiredto register the circuit-switched controlled user's terminal in the IPmultimedia system.
 3. The Access Gateway Node according to claim 1,wherein the exchange interface is configured to forward the user's IPmultimedia system parameter from the circuit-switched function to theuser agent.
 4. The Access Gateway Node according to claim 1, wherein theexchange interface is configured to deliver an indication to the useragent to register the circuit-switched controlled user's terminal in theIP multimedia system or to update the registration of thecircuit-switched controlled user's terminal.
 5. The Access Gateway Nodeaccording to claim 1, wherein the exchange interface is configured totranslate between circuit-switched controlled signaling and IMScontrolled signaling.
 6. The Access Gateway Node according to claim 1,wherein the circuit-switched function is configured to receive from amobile switching node to which the circuit-switched controlled user'sterminal is attached, a mobility related notification to provision anaccess to the IP multimedia system.
 7. A register node located in a homecircuit-switched controlled domain for communicating with acircuit-switched function, the register node comprising: a receiverentity configured to receive from the circuit-switched function, arequest message to handle a circuit-switched controlled user's terminal;a provision module configured to provide information about afunctionality of the circuit-switched function sending the requestmessage, wherein the functionality indicates whether thecircuit-switched function is part of an Access Gateway Node forproviding an access to the IP multimedia system; a user's parametermodule configured to provide a user's IP multimedia system parameterapplicable in the IP multimedia system when the circuit-switchedfunction is part of the Access Gateway Node; and a sender entityconfigured to send a message to the circuit-switched function with theuser's IP multimedia system parameter, the user's IP multimedia systemparameter being an indication that a subscription of thecircuit-switched controlled user's terminal has been ported to the IPmultimedia system; wherein a user agent in the Access Gateway Nodeutilizes the user's IP multimedia system parameter to register thecircuit-switched controlled user's terminal in the IP multimedia systemon behalf of the circuit-switched controlled user's terminal, whereinthe user agent registers the circuit-switched controlled user's terminalin the IP multimedia system without further signaling with thecircuit-switched controlled user's terminal.
 8. The Register nodeaccording to claim 7, further comprising a list with circuit-switchedfunctions with the information about the supported functionality,wherein the provision module has access to said list.
 9. The Registernode according to claim 7, wherein the information about thefunctionality is included in a notification message provided from thecircuit switched function, and the provision module is configured toextract said information.
 10. The Register node according to claim 7,wherein said register further comprises a tracking module configured tokeep track of the serving Access Gateway Node and of a mobile switchingnode to which the circuit-switched controlled user's terminal isattached.
 11. The Register node according to claim 7, wherein saidregister further comprises a selection unit configured to select a newserving Access Gateway Node when the circuit-switched controlled user'sterminal leaves a serving area of a present Access Gateway Node.
 12. TheRegister node according to claim 7, wherein the user's parameter modulefurther comprises a modification module configured to modify the user'sIP multimedia system parameter before sending it, when thecircuit-switched controlled user's terminal is served by a mobileswitching node.
 13. The Register node according to claim 12, wherein themodification module is configured to remove services from the user's IPmultimedia system parameter which are handled in the IP multimediasystem.
 14. The Register node according to claim 7, wherein the user'sIP multimedia system parameter is provided to the user's parametermodule either after migration of a user to the IP multimedia systemperformed by an operator or after an explicit registration of a user tothe IP multimedia system.
 15. A method of providing access to an IPmultimedia system for a circuit-switched controlled user's terminallocated in a circuit-switched controlled domain, the method comprisingthe following steps: a circuit-switched function being part of a AccessGateway Node receiving from a circuit-switched controlled user'sterminal, a registration message; the circuit-switched functioninteracting with a home register node located in a home circuit-switchedcontrolled domain to obtain from the home register node, a user's IPmultimedia system parameter, the user's IP multimedia system parameterbeing an indication that a subscription of the circuit-switchedcontrolled user's terminal has been ported to the IP multimedia system;the circuit-switched function providing the received user's IPmultimedia system parameter to an user agent being part of the AccessGateway Node; and the user agent utilizing the user's IP multimediasystem parameter to register the circuit-switched controlled user'sterminal in the IP multimedia system on behalf of the circuit-switchedcontrolled user's terminal, wherein the user agent registers thecircuit-switched controlled user's terminal in the IP multimedia systemwithout further signaling with the circuit-switched controlled user'sterminal.
 16. A method of providing access to an IP multimedia systemfor a circuit-switched controlled user's terminal located in acircuit-switched controlled domain with the following steps to beperformed in a home register located in a home circuit-switchedcontrolled domain and configured to communicate with a circuit-switchedfunction: receiving a request message regarding a circuit-switchedcontrolled user's terminal from the circuit-switched function forhandling the circuit-switched controlled user's terminal; providinginformation about a functionality of the circuit-switched functionsending the request message, wherein the functionality indicates whetherthe circuit-switched function is part of an Access Gateway Nodeproviding access to the IP multimedia system; providing a user's IPmultimedia system parameter applicable in the IP multimedia system whenthe circuit-switched function is part of the Access Gateway Node; andsending a message to the circuit-switched function with the user's IPmultimedia system parameter, the user's IP multimedia system parameterbeing an indication that a subscription of the circuit-switchedcontrolled user's terminal has been ported to the IP multimedia system;wherein a user agent in the Access Gateway Node utilizes the user's IPmultimedia system parameter to register the circuit-switched controlleduser's terminal in the IP multimedia system on behalf of thecircuit-switched controlled user's terminal, wherein the user agentregisters the circuit-switched controlled user's terminal in the IPmultimedia system without further signaling with the circuit-switchedcontrolled user's terminal.